1. Field of the Invention
The present invention relates to a method for generating a mask pattern.
2. Description of the Related Art
According to miniaturization of a semiconductor device in recent years, it is difficult to transfer a micromask pattern of a mask using an exposure apparatus. Therefore, a technology that performs an optical proximity correction (OPC) on the mask pattern or provides an auxiliary pattern to improve a resolving property of the mask pattern is known. Here, the resolving property refers to a property regarding resolution, such as a contrast of an image, a normalized image log slope (NILS), a depth of focus, a process window, an exposure margin. The auxiliary pattern is also referred to as an assist feature (AF), a sub-resolution assist feature (SRAF), or a scattering bar (SB). The auxiliary pattern is provided separately from a main pattern to improve the resolving property of a main pattern to be transferred to a wafer.
Japanese Patent Application Laid-Open No. 2009-093138, U.S. Pat. Nos. 7,979,812 and 8,099,684 discuss a determining method for an auxiliary pattern. Japanese Patent Application Laid-Open No. 2009-093138 discusses a method for determining a position of an auxiliary pattern using an approximate aerial image. Japanese Patent Application Laid-Open No. 2009-093138 does not discuss whether to transfer the auxiliary pattern itself. U.S. Pat. No. 7,979,812 discloses a method for, when an auxiliary pattern is transferred to a wafer after OPC, performing the OPC again around an area, on which the auxiliary pattern is to be transferred, and determining the auxiliary pattern so as not to completely transfer the auxiliary pattern. U.S. Pat. No. 8,099,684 discusses a method for determining an auxiliary pattern, which is used for first exposure, in double exposure, to transfer the auxiliary pattern to a wafer in the first exposure but removes the transferred auxiliary pattern by second exposure.
In the invention discussed in U.S. Pat. No. 7,979,812, a main pattern to be transferred to a wafer is transferred but an auxiliary pattern is limited so as not to be completely transferred, so that an effect of improving a resolving property of the main pattern by the auxiliary pattern is limited. According to a review of the present inventors, if a size of the auxiliary pattern is increased, the resolving property of the main pattern is improved in many cases. However, if the size of the auxiliary pattern is too large, the auxiliary pattern itself is transferred to the wafer as discussed in U.S. Pat. No. 8,099,684.
Here, it is assumed that a pattern is transferred by exposure to a wafer having a laminated structure, in which a pattern is already formed on a lower layer and a resist is applied thereon, and processing is performed on the pattern on the lower layer using the transferred pattern as a mask (a cover layer). In this case, if the auxiliary pattern is transferred by the exposure, the lower layer is processed according to a shape or a position of the transferred auxiliary pattern. If a part of the lower layer which is processed by the auxiliary pattern coincides with a part which should not be processed, that results in a defect due to a structure of the pattern of the lower layer, and may be a problem. However, if the part of the lower layer which is processed by the auxiliary pattern does not coincide with the part which should not be processed, there may not be a problem in some cases. In other words, depending on the relationship between the position of the pattern of the lower layer and a transfer position of the auxiliary pattern, even though the auxiliary pattern is transferred, there may not be a problem.
In the invention discussed in U.S. Pat. No. 8,099,684, even though the auxiliary pattern is transferred by the first exposure, the auxiliary pattern does not remain on the wafer while being transferred thereto by performing double exposure, so that it is not assumed that the pattern of the lower layer is processed by the auxiliary pattern. In other words, the auxiliary pattern is determined without considering the pattern of the lower layer.
As described above, in related art, when the auxiliary mask pattern is determined, a method for generating the auxiliary pattern while considering the relationship between the position of the pattern of the lower layer and the transfer position of the auxiliary pattern has not been known.